Imagine a typical fantasy world. Elves, goblins, dragons, magic. If you were to equate it to a D&D campaign, you would be close enough for proverbial government work. That said, thanks to time and magic, their knowledge of science is more advanced than usual. The people understand germ theory, and atoms (though not sub atomics).

One exception is that, in this world, it is popular to quench newly forged swords in dragon's blood instead of oil (or gods forbid, water). This is not just mysticism or cruelty, doing so produces demonstrably superior weapons. And it is not a magical benefit, dragon's blood swords are still better without any detectable powers or while in an anti-magic zone.

Why would that be? What physical properties could dragon's blood possess, that would somehow make it better for heat treating blades? And while still being viable blood for a living creature?

$\begingroup$What qualifications are used to consider a sword "better" if it is treated with blood?$\endgroup$
– BewildererMar 1 at 1:50

$\begingroup$Are we free to alter the chemical composition of dragons' blood to whatever we need it to be? For instance, making it more caustic? Acidic? Iron content? Boiling point?$\endgroup$
– Justin ThymeMar 1 at 2:10

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$\begingroup$@Bewilderer Any improvement without a noticeable detriment. Can be sharpened to a narrower edge and hold it better without becoming brittle. Can flex better with losing strength. Whatever would be viable improvement, and is scientifically possible.$\endgroup$
– Xavon_WrentaileMar 1 at 3:30

$\begingroup$@JustinThyme Certainly, so long as it still works as blood.$\endgroup$
– Xavon_WrentaileMar 1 at 3:31

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$\begingroup$Are we assuming that blacksmiths just have vats of dragon blood on-hand? How long does it keep? Are dragons common enough that you can maintain a workable supply of the stuff? Can you improve on your regular sword by sticking it into a dragon? (And how do you trick a dragon to get close enough to your forge to be able to do that before it cools naturally?)$\endgroup$
– Darrel HoffmanMar 1 at 18:56

15 Answers
15

You don't want harder steel, it breaks, you don't want softer steel, it bends, you want more control of the temperature of the steel at every stage so you can get the exact properties you want. Better is more control of the process.

Dragon blood is just a non-flammable liquid with thermal conductivity similar to oils or water but unlike water and oils it has a vapor point higher than steel's melting point, so the oil can be heated to much higher temperatures. This means it can control the temperature the steel is cooled to exactly.

This also allows it to be used in a liquid oven or normalizing medium, which means the steel can be heated perfectly evenly and kept there as long as necessary. This is how the best steels are produced and how to get the most out of your steel allows, precision control of temperature at every stage. It can even be used for annealing and tempering much how modern smiths use ovens.

Most forges and quenches can not be targeted to the exact temperature, quenching fluid in particular boils off before it can be heated to the best temperatures. It is really easy to overheat or underheat the steel. Today we can use molten salts for some of these, but that is a recent invention. Molten salt is also extremely dangerous.

Making good steel is about precision, you are trying to hit a bullseye of qualities. For those unfamiliar with the term ductility, low ductility means brittle, high ductility means easy to bend, you don't want either.

It even makes sense that dragons would have such blood since they need blood that will not boil no matter how hot they are.

$\begingroup$awesome answer. The grain type shown in the recovery part of that graphic reminded me of a chart i saw related to magnetism and this gives me an idea, what if the blood is magnetic for whatever reason? while the steel is easily manipulable the magnetism would align the grain to a more perfect form right?$\endgroup$
– Elias Rowan AlbatrossMar 1 at 16:52

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$\begingroup$imagine a slow, magnetized cooling. making a blade that is perfectly aligned and well hardened, and then gilded with arsenic making it poisonous to any who are stabbed by it. i feel like something can be taken from all these answers$\endgroup$
– Elias Rowan AlbatrossMar 1 at 16:54

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$\begingroup$@EliasRowanAlbatross Poison is more or less pointless on a sword, especially arsenic, you need to take in a lot of arsenic to bother someone. Being magnetically aligned is probably a net bad,getting stuck to you opponent sword or armor might get you killed.$\endgroup$
– JohnMar 1 at 16:58

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$\begingroup$magnetised steel doesnt remain magnetic forever. you can demagnetise it. it will just have a well aligned grain that absorbs shock better than other swords and would thus not break as easily.$\endgroup$
– Elias Rowan AlbatrossMar 1 at 17:02

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$\begingroup$@EliasRowanAlbatross I Thought you meant permanently magnetized, I see what you mean, yeah that could be beneficial.$\endgroup$
– JohnMar 1 at 17:05

It generally doesn't matter what type of fluid you quench your blade in. The general thing to know is that the faster the steel cools down from critical (the point where it is so hot it is no longer magnetic) the harder and more brittle it becomes.

Brief definition break!

Hardness: Hardness is a measure of how much force it takes to deform the steel

Toughness: Toughness is a measure of how much force a blade can take and still return to is previous shape.

OK so anyway. You want the edge of the blade to be hard and you want the spine of the blade to be flexible. To do this you first harden the blade by quenching it as mentioned.

Once that is done you reheat the blade but only along the spine back to a cherry red, say 800 degrees or so. While you do this you keep the cutting edge cool either by keeping it in liquid or wet clay or something, this keeps the edge hard.

OK. On to the question at hand...how does dragon blood make blades better?

Sadly, with science...it doesn't. When you quench the blade in any substance, the outer layer will indeed bond with the quenching fluid (we use a mix of motor oil and antifreeze at my shop). The problem is, it is a very thin layer, and depending on the type of steel may actually flake off. Either way you would grind and polish after hardening. Then you would temper the spine. After tempering you do your final grind/polish and sharpening.

Metallurgy really doesn't allow for a scientific benefit to using dragon blood...which is great for your dragons.

That being said if you are working in a fantasy realm...which I would assume you are since...you know...dragons, you can use the ol' it's magic and create something that works for story telling. Just because it doesn't work in real life doesn't mean it can't be awesome in a story.

$\begingroup$@John and I explained that is unfortunately not possible, but wanted to suggest that the magic option in a world that already has dragons is still pretty awesome.$\endgroup$
– James♦Mar 1 at 17:05

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$\begingroup$You might want to read my answer. It could be beneficial if it has the right properties. Basically molten salt, without all the problems molten salt has. Nice to meet a fellow smith.$\endgroup$
– JohnMar 1 at 17:09

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$\begingroup$@John The end result might be non-magical, but the property of the Dragon Blood could be - perhaps it reacts endothermically with the steel to form a magically heat-resistant coating - then you can scour the spine of the blade clean, and heat it to cherry-red while leaving the cutting edge cold for a superior temper?$\endgroup$
– ChronocidalMar 1 at 17:16

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$\begingroup$We may have to talk blade making in chat, I am curious which blades you wouldn't want to differentially treat. Feel free to join me in there.$\endgroup$
– James♦Mar 1 at 17:34

Dragons' blood is an absurdly good insulator. Now, normally this would not be quite what you want for quenching, but Dragons' blood is also magical.

Quenching a blade in Dragons' blood is not like a normal quenching - it's more like aging a whiskey. You heat the blade to white-hot, plunge it into the blood, seal it up, and leave it for a month or so.

The blade then cools incredibly slowly. Aided by the intrinsic magic of the blood purifying the lattice and eliminating dislocations, the end result is a nigh-indestructible monocrystalline blade which requires no further tempering.

The blade must then be sharpened by magic, resulting in this monomolecular blade having an edge only a handful of atoms thick - but despite the magic involved in the forging process, no actual magical properties are imposed on the sword itself.

(This also explains why Dragons sleep on hoards of gold and treasures made from other metals - the magic in their blood interacts with the metal, giving them the same basking feeling that a lizard gets in the hot sun)

$\begingroup$@GarrettMotzner A flawed monocrystal with dislocations certainly wouldn't - excessive stress could cause propagation of dislocations and generate a grain boundary - but a flawless monocrystal would lack those 'nucleation points' for the weakness to occur at.$\endgroup$
– ChronocidalMar 2 at 11:36

Phosphorus in steel can have beneficial as well as harmful effects.
Phosphorus is one of the most potent solid-solution strengtheners of
ferrite. The addition of only 0.17% phosphorus increases both the
yield and tensile strength of low-carbon sheet steel by about 62 MPa
(9 ksi) while also improving the bake hardening response and deep
drawability... Phosphorus is also used as an additive in steels to
improve machining characteristics and atmospheric corrosion
resistance.

You do not want phosphorus mixed with your steel. It will make it brittle. You want a thin layer on the outside. That hardens the outer layer, where you want it to hold and edge. Also that outer layer offers corrosion resistance, which you do not need on the sword interior.

Your question does not state that the swords are steel. Maybe they are bronze. Phosphor bronze.

Phosphor bronze is an alloy of copper with 0.5–11% of tin and
0.01–0.35% phosphorus. The tin increases the corrosion resistance and strength of the alloy. The phosphorus increases the wear resistance
and stiffness of the alloy.

If you had Bronze Age tech, phosphor bronze would be excellent sword making stuff. Wikipedia shows a phosphor bronze ship propeller - for tool making applications this would be great, and a dip in high phosphorus dragon blood would be a way to get a layer of phosphor bronze on the outside of your bronze weapon.

I got this idea because I thought I had read that the druids did exactly this with swords and human blood - quenched them in blood to harden the outside, which was accomplished by the high phosphorus content of blood. Animals run on ATP which is a high energy phosphorus compound and so all blood has a lot of phosphorus. You could make dragon blood exceptionally high - perhaps they need a lot of circulating ATP to produce fire.

$\begingroup$quenching a sword in a phosphorus will have zero effect on the phosphorus content, its not hot enough to be incorporated, and any thin crust will be ground off during sharpening. but good point about a bronze sword being possible.$\endgroup$
– JohnMar 1 at 16:02

$\begingroup$@John Dragon blood is not pure phosphorus. Aren't there any additives that would make it happen in quenching temperatures?$\endgroup$
– MołotMar 1 at 17:42

For those of us who are not up-to-date on their material science, quenching has the end effect of making metal harder. It achieves this through the cooling rate of the metal. It can be that dragon's blood happens to conduct heat really well, making a harder blade.

This may also explain why those darn dragons don't burn themselves so easily : their blood helps dissipate the heat so well!

Additionally, cooling some metal quickly enough can result in it being amorphous. In generalities, amorphous metals (or glass metals) resist plastic deformation and are tougher than crystalline (normal) metals. Tougher blades, though, are generally considered better- they snap back better to their original shape and don't become bent as easily.

Harder blades are not always better: it depends on the style of sword and martial system you are using. Additionally, increased hardness usually comes with increased brittleness, increasing the chances of a blade shattering.

If it can cool metal weapons quickly enough and achieve the "glass metal" mentioned above, it will be tougher which will result in a better blade.

Dragon's blood is an ideal medium for austempering

While most blades, to this day, are made using water or oil as the quench medium in the traditional quenching and tempering process that James mentions, producing a tempered martensite microstructure, this is actually not the ideal microstructure for a given hardness. The work of Bain and Davenport at US Steel in the 1920s and 1930s (pat. 1933) on isothermal transformations in steel led to the discovery of a superior microstructure, namely bainite, with improved toughness at a given hardness for typical blade hardnesses (above 40 on the Rockwell C hardness scale). However, it isn't achievable in ordinary carbon steel using typical, continuous-cooling quench media. Modern production dunks the part in a molten salt (nitrite/nitrate) bath that cools the part to an isothermal transformation temperature, then holds it there to effect the transformation before pulling it out and letting it air-cool post-austemper, or uses special alloys that can form bainite during a continuous cooling process. This is known as austempering, and is commonly used for high-strength steel parts such as rifle bolts (all the way back to WWII), mower blades, and seat belt parts in cars.

In your case, though, you can do better. The blood of your dragons is a high-boiling liquid (very low vapor pressure) with excellent thermal stability, excellent thermal conductivity, and a high specific heat capacity, making it ideal for austempering a blade as it will not boil off, decompose, react with the blade, or change in temperature much when the part is added, while drawing heat out of the part quickly to bypass the "nose" of the curve for that steel. In this process, the quench tank would be almost like a cauldron, kept hot (but not too hot!) with a stoked fire, and the parts would be held in the quench medium for a significant length of time, effecting an isothermal austemper and producing tougher, stronger blades for a given hardness at a minor tradeoff in absolute hardness achievable. Once done, the part would be removed from the bath, washed, and for a blade taken straight to the grinding wheel for sharpening, as austempered parts need no further heat treatment.

$\begingroup$+1, What I would have gone for. Austempering really seems to be the only realistic option. Although I was thinking along the lines of the blood actually boiling. At precisely the correct temperature. This means it can be easily kept at that correct temperature for whatever time you need. As long as there is blood in the "kettle" it cannot overheat and as long as it is slowly boiling it is hot enough. Since dragon blood is probably rare you might want a system that collects the "steam", condenses it and returns it back. Boiling water has been used for temperature control for ages.$\endgroup$
– Ville NiemiMar 10 at 4:04

$\begingroup$@VilleNiemi -- that could work for temperature regulation as well, but would have some disadvantages in terms of getting parts in and out of the bath. (You'd also have the composition of the quench bath changing, which probably doesn't matter too much, but could be a problem...?)$\endgroup$
– ShalvenayMar 10 at 4:07

$\begingroup$It will be a pain anyway. But with proper timing and sizing you can make it so that the blood has just stopped boiling before you put the parts in or take them out. Still I am guessing you'd want some specialized tool that lets, say two people, to carry the blade, put it in and take it out while maintaining a reasonable distance. Or a simple hang blade here and turn the hand crank until the blade is in the blood type of machine.$\endgroup$
– Ville NiemiMar 10 at 4:13

$\begingroup$True about the composition changing. It won't matter if can recondense the steam back or replenish the blood for the duration and the vessel is of sufficient size. You could also just use distilled dragon blood. It would already have all the potentially problematic stuff removed.$\endgroup$
– Ville NiemiMar 10 at 4:18

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$\begingroup$Off topic. I really love worldbuilding. Where else can you have a conversation about the practical uses of distilled dragon blood?$\endgroup$
– Ville NiemiMar 10 at 4:20

Perhaps the dragon's blood is high in elements which are inherently poisonous to humans. A thin coating of mercury or arsenic on a blade can dramatically increase the killing effectiveness of the steel. Add to that real threat, the psychological burden of knowing that your enemy's blade is poisoned, and battles can be won before the blade is even drawn.

Now go deeper than just a thin coating...

Perhaps quenching a scalding hot blade in poisonous metals saturates the resulting steel with a lethality which can't just be wiped away. It is in the metal and will be the deciding factor of every battle in which the blade is used.

Alternatively, the poison could be biochemical. Perhaps the dragon's blood contains a voracious infection which thrives in the scalding bloodstreams of dragons. When stored in the structure of cold steel, the microscopic life lays dormant until revived by hot human blood. Once activated, it quickly consumes the victim since human immune defenses are no match for a virus born in dragons. This alternative has the advantage that the blade could be handled, cleaned and cared for as long as it never came in touch with blood. It would be safer for it's wielder to carry than the elementally poisoned blade described above.

$\begingroup$You can't incorporate anything into the steel by quenching it it, if you added it while the steel was still molten you might be able to incorporate it, but steel is not liquid enough to diffuse into during a quench.$\endgroup$
– JohnMar 1 at 16:50

Quenching a sword in blood imbues the properties of the blood (and by extension the animal from which the blood is drawn) into the sword. Human blood? Actually pretty bad, since people whose blood is being used to quench the sword are obviously LOSERS! Rhinoceros blood? Great strength/crappy eyesight - blade won't ever break, but you can't hit sh*t with it. Bats blood? Makes for a nice, lightweight blade, very quick and easy handling, but it squeeeeeeaks every time you draw it out of the sheath - puts everyone's teeth on edge, and obviously no good for a ninja. Horse blood? Leaps into your hand, but really touchy about what it eats/hits - insists on second-cutting hay, gets colic-y at the drop of an anvil, and is prone to develop hoof/grip problems. Snakes blood? Poisonous, and likes to bite the hand that wields it. NOT GOOD! Lizard blood? Slow, and intolerant of cold weather.
Dragon's blood? Well, now - that's a bit of a poser. Y'see, according to a handwritten marginal note found in Nurmenfnograth's Tome Of Magical Metallurgy & Metalworking:

The entire book was found in a rather scorched-around-the-edges condition outside of The Great Smoking Cave. Nurmenfnograth was never seen again, and the only clue as to his fate was a brief and cryptic inscription found scratched into the bare rock next to the book in the Ryunes Of Gnobrathmingfnorp, which translates rather loosely as

NEXT!

None of The Wise who have been consulted regarding this matter can explain quite what this inscription means. ???

Quenching is the process in which steel is cooled from extremely high temperatures to not so high ones. the faster the cooling the harder the steel. According to a few pages I just read nothing is faster at dispersing thermal energy from other materials than water.

So in order to make dragons blood superior for quenching to water, we need to find its one flaw. that is water cools steel so quickly that it doesn't cool evenly which is absolutely necessary to create perfect steel.

So all you need to do to make your dragon blood so amazing is have the perfect ratio of water to any other materials in the blood to make the cooling faster than that of oil or salt bathing, but not as fast as water. You need no special chemical properties or materials, just the right ratio of water to not water. I hope this makes sense and is helpful. Thank you :)

$\begingroup$Brine cools steel faster than normal water, faster cooling is not good for a sword, it makes steel brittle.$\endgroup$
– JohnMar 1 at 15:02

$\begingroup$@John "make the cooling faster than that of oil or salt bathing, but not as fast as water." Nowhere said faster than water. It's "better than water through a better ratio and cooling process due to chemical properties"$\endgroup$
– NyakouaiMar 1 at 15:11

$\begingroup$Even oils cool too quickly for ideal results, there is no upside to cooling faster. You are not going to be using dragon blood to add carbon, you can already mine graphite and more importantly make charcoal.$\endgroup$
– JohnMar 1 at 15:23

$\begingroup$@John you did worry me that I had made a mistake so I took your wisdom and did a little more research. here's what i found, please take a look: " Water is the quickest quenching method." - jfheattreatinginc.com/2017/01/… "brine wets the metal surface and cools it more rapidly than water" - quora.com/… it seems something that should be basic fact has been turned into somewhat of an opinionated matter.$\endgroup$
– Elias Rowan AlbatrossMar 1 at 16:11

Dragons are fierce creatures the size of small commercial jets. The forces involved when they fight are intense. Consequently, they need many unique adaptations just to avoid bleeding to death or becoming permanently disabled in the course of their frequent scuffles.

One of those adaptations is an advanced clotting factor in their blood. When exposed to intense heat it forms a strong impermeable coating film w/ over 2-3x the toughness & 3-6x the tensile strength by weight of spider silk (at a density comparable to that of steel). Quenching in Dragon's Blood thus deposits a microscopic coating that happens to bond to steel really well (due to little understood adaptations for rapid bone regeneration that apparently rely on high concentrations of several organometallic compounds dissolved in their blood) and which far exceeds the performance of any synthetic materials we can hope to make.

$\begingroup$Interesting, but the problem is the sharpening. If it is too strong, it would make it really difficult to sharpen the blade. And if the coating is too thin, sharpening would just take it off.$\endgroup$
– Xavon_WrentaileMar 2 at 15:50

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$\begingroup$@Xavon_Wrentaile on the plus side of things, even if sharpening wore off a bit on the edge, rust would be a non-issue for most of the blade surface$\endgroup$
– MorgenMar 3 at 9:18

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$\begingroup$that's a good point. the way I imagined it, you'd want the blade razor sharp before the final dip in dragon's blood, & afterwards you wouldn't be able to sharpen it per se, but the coating would be so thin & abrasion resistant that you would only want to hone or polish it the smallest amount anyway. if it eventually needed real sharpening, it would mean the coating had failed so they'd have to redo the finishing & coating steps$\endgroup$
– Nathan SmithMar 4 at 4:42

It's a little-known fact that dragon's blood has an exceptionally high graphite content, a byproduct of their evolution from the igneous realms of the molten hearts of mountains. Graphite is a blacksmith's friend for three reasons. First, being elemental carbon, it can be used in the production of weapon-grade steel. One fist of dragon's blood for every twelve fists of molten iron has been found to be the ideal ratio for the hardest "dragon steel". Second, graphite is an excellent refractory material able to provide stable heat insulation across a wide range of temperatures. When quenching hot steel in dragon's blood, the liquid effectively becomes a kiln that cools weapon steel evenly and, by coincidence or godly design, at the ideal rate that ensures maximum hardness. Finally, graphite is a superb lubricator, and the use of dragon's blood while grinding a dragon steel blade is known to yield the sharpest possible edge.

Well cooling metal too slowly results in it being too soft but cooling it too fast results in it being hard but brittle. Dragons blood you see, it contains metals that get heated when it breaths fire and then they are cooled in a controlled way by the blood. The result is that these metals are cooled not too fast, not too slow but just right. so that it can use them in its scales. dragon blood has evolved to do this. when you quench metal in this blood you will benefit from its properties.

Dragons are usually big and armored, begging the question of where the square cube law was when they were created. But there are solutions. As material sciences advance we notice that molecularly perfect substances often exibit extreme proporties, so we can only assume that Dragons have evolved to create many of these materials to strengthen themselves and make them able to support their own weight.

Damascan steel managed to get very strong and still elastic through nanowires and carbon Nanotubes that were enclosed in the metal. Such materials could be present in dragon's blood especially if parts of the dragon were ground into the blood.

What happens during the quenching is that these materials coat the blade, providing a superior resiliance to shattering, dentation and needing less effort to remain sharp. If you then use folded steel and quench between foldings you create many layers of this coating inside the blade, making them have superior properties in strength, resilience, maintenancr and resistance to damage/shattering.

$\begingroup$As with most physical laws, in the presence of a sufficiently dense magical field the square-cube law decides that it's time to go out for a smoke and perhaps a pint down at the pub, and leaves the world (and the dragons) to their own devices.$\endgroup$
– Bob JarvisMar 4 at 13:46

Historically slaves and prisoners were used to quench some blades.
Something I heard in a Materials Science class, supposedly the Conquistadors' blades were quenched by running a captive Inca through with a blade to quench the blade. Something about the body temperature of a person, plus the saline content allowed for just the right speed of blade temperature decrease.

With that in mind, suppose you had a different alloy, one that under normal forging processes would either be too expensive for your average customer to buy, or too heavy for them to wield. But then with using dragon's blood to quench the blade at just the right temperature and speed, you can make your blade stronger but lighter, and thereby use fewer metallic resources, and thereby bring the finished blade into the wieldable and affordable range.

There are already enough answers which touch on how the blood could magically make the sword better and I think James' answer sums it up the best in terms of real-world viability and properties.

I offer you another reason why these swords are perceived as "better" and that reason is psychology.

Imagine facing an army who's reputation is:

You mean we are fighting the XYZ's? The nation that forges their swords in the blood of dragons and then quenches the sword in the blood of their enemies? Peace out guys, I'll pass.

If the wielders are brainwashed into thinking their swords are better through questionable demonstration of a blood-sword versus a purposely built brittle sword then their confidence in battle will simply be higher. This equates to better morale and overall greater fierceness of the warriors. This of course assumes that the blood swords are forged to a high quality per James' answer.

You don't have to look much further for this placebo than the Rhino Horn.